The present invention relates to the control of internal combustion engines. More specifically, the present invention relates to a method and apparatus to control a variable displacement internal combustion engine.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle. Variable displacement internal combustion engines (ICEs) provide for improved fuel economy and torque on demand by operating on the principal of cylinder deactivation. During operating conditions that require high output torque, every cylinder of a variable displacement ICE is supplied with fuel and air (also spark, in the case of a gasoline ICE) to provide torque for the ICE. During operating conditions at low speed, low load, and/or other inefficient conditions for a fully displaced ICE, cylinders may be deactivated to improve fuel economy for the variable displacement ICE and vehicle. For example, in the operation of a vehicle equipped with an eight-cylinder variable displacement ICE, fuel economy will be improved if the ICE is operated with only four cylinders during low torque-operating conditions by reducing throttling losses.
Throttling losses, also known as pumping losses, are the extra work that an ICE must perform to pump air from the relatively low pressure of an intake manifold through the ICE and out to the atmosphere. The cylinders that are deactivated will not allow air flow through their intake and exhaust valves, reducing pumping losses by forcing the ICE to operate at a higher intake manifold pressure. Since the deactivated cylinders do not allow air to flow, additional losses are avoided by operating the deactivated cylinders as xe2x80x9cair springsxe2x80x9d due to the compression and decompression of the air in each deactivated cylinder.
During the operation of the ICE, if the ICE is in a condition where it is inefficient to operate with the full complement of cylinders, a controller will deactivate the mechanisms operating the valves for selected cylinders and also shut off fuel to the cylinders. During a high torque demand condition for the ICE, the cylinders will be reactivated by operating the valves and supplying fuel to the cylinders.
The present invention is a method and apparatus for the control of cylinder deactivation in a variable displacement engine. In the preferred embodiment of the present invention, an eight-cylinder internal overhead valve (OHV) combustion engine (ICE) may be operated as a four-cylinder engine by deactivating four cylinders. The cylinders are deactivated by an electro-hydraulic deactivation system using engine oil pressure, controlled by solenoids, to pressurize the locking pins of special switching engine valve lifters (as used in an OHV engine). With pressure applied, the locking pins disengage and allow the lifter to act as a lost motion device to prevent exhaust and intake valve activation.
A problem associated with hydraulic devices and control devices in general is the response time. The device inertia, hydraulic line flex, the movement of hydraulic fluid, and other physical performance characteristics create a delay between the commanded hydraulic control and the actual response. The delay may create problems with synchronizing cylinder deactivation, reactivation and engine control events. Proper synchronization of the cylinder deactivation system is necessary to ensure the exhaust lifter is always deactivated or reactivated prior to the intake valve deactivation or reactivation, respectively. Deactivation and reactivation of a cylinder/valve must occur within a small crank angle window to insure the exhaust valve state change always leads the intake valve state change. Improper timing can result in poor driveability, increased emissions and/or mechanical problems.
The control system of the present invention generates response time estimates for the system based on the physical properties of the solenoid valve, hydraulic system, lifter and other devices used in cylinder deactivation to accurately control the solenoid timing. Test or timing data is generated from a simulation of a physical model of the system and testing of the solenoid, hydraulic interface and lifter on a hydraulic test fixture with adjustable pressure, temperature, and voltage. A response time estimation is then generated based on the test data and physical properties of the devices involved during the cylinder deactivation/reactivation.
The precise timing for solenoid control is delivered by a timer processor unit (TPU) or similar device with output compare timers based on camshaft and crankshaft reference position detector inputs. Solenoid timing is controlled by an event based state machine. A state machine is a representation of an event-driven (reactive) system. In an event-driven system, the system transitions from one state (mode) to another prescribed state, provided that the condition defining the change is true. The state machine uses an array of records stored in a controller for valve status and solenoid status. The valve status record includes the valve mode (enabling, disabling, disabled, enabled), events from target angle, injector state change cylinder event, air model transient mode cylinder event, air model calculation change cylinder events, and fuel injector status. The solenoid status record includes the target angle, start angle, start cylinder event, previous start cylinder event, TPU offset time, previous TPU offset time, and solenoid control mode.
The cylinder deactivation in the present invention occurs as a function of load or torque demand by the vehicle. An engine or powertrain controller will monitor vacuum pressure/torque and determine if the ICE should enter four-cylinder mode. Fuel economy for a variable displacement ICE is maximized by operating in a partially-displaced mode or configuration. The present invention reduces mistiming errors when deactivating and reactivating cylinders to protect the engine valve lifters and maximize the amount of time spent in a partially-displaced operation while maintaining the same performance and driveability of a fully-displaced ICE.